1. Field of the Invention
The present invention relates to the production of ethyl tertiary butyl ether (ETBE) from the reaction of isobutene (iC.sub.4.sup.=) with ethanol (EtOH). More particularly the invention relates to a process using a primary straight pass reactor in combination with a catalytic distillation column reactor.
Related Art
The production of MTBE from the acid catalyzed reaction of iC.sub.4.sup.= and MeOH is well known in the art. Generally the iC.sub.4.sup.= is contained in a mixed hydrocarbon stream containing predominantly C.sub.4 's which includes normal butenes, butanes and possibly lighter C.sub.3 hydrocarbons. The iC.sub.4.sup.= content of these streams is typically from 10-70 mole %. The MeOH preferentially reacts with the iC.sub.4.sup.= to form MTBE with the remainder of the materials in the mixed hydrocarbon passing through essentially as inerts. The use of catalytic distillation particular method is exemplified by U.S. Pat. Nos. 4,232,177; 4,307,254; and 4,336,407 and the combination of the straight pass reactor and the distillation column reactor is disclosed in U.S. Pat. No. 4,950,803.
These ethers are produced in vast quantities throughout the world and employed as combustion and octane improvers in gasoline. Methanol is very easily used in the combined reactor system described in U.S. Pat. No. 4,950,803, which is the system of choice by most new facilities. Until recently methanol has been used in the commercial units; however, the high demand for the ethers has reduced the amount of methanol available and increasing the cost. There are recognized problems with the use of methanol in refinery systems for gasoline components, since the methanol must be essentially excluded from the gasoline, because of its tendency to phase out.
Ethanol, although it has some undesirable properties in gasoline, is now approved as a gasoline component and does not phase out as readily as methanol. Much ethanol that is available and will be available is made by fermentation of grains, and must be separated from the aqueous fermentation broth, normally by distillation. Unfortunately, water and ethanol form an azeotrope containing about 4 or 5 weight percent water, so that it is necessary to break this azeotrope using a two column system, in which the water is removed by a third component such as cyclohexane. The capital and operating cost of the azeotrope breaking systems adds to the cost of producing ethanol, which is an already expensive component for gasoline use. It would be better for ether production to use 95% "wet" ethanol; however, in all of the MTBE and ETBE syntheses, using the acid resin catalyst the presence of water can result in the production of tertiary butyl alcohol while also reducing the activity of the catalyst.
In the MTBE reaction using the combined reactors the isobutene containing feed and the methanol are fed as a mixed stream to the straight pass reactor, where about 85-90% of the isobutene is reacted, and the reaction product from the straight pass reactor is fed below the catalyst bed in the distillation column reactor, where the major portion of the unreacted isobutene is reacted to give total yield of 97+%. When this system is used for ETBE production the ethanol forms minimum-boiling azeotropes with the C.sub.4 hydrocarbons at a concentration of about 2 weight percent at the operating conditions for the etherification. The C.sub.4 distillate from the distillation column reactor contains the excess ethanol, but in many cases the ethanol carried into the catalytic distillation structures is not sufficient to react with the unreacted isobutene from the straight pass reactor. One solution is to feed more ethanol directly to the catalyst bed in the column; however, because of the limited amount of ethanol which could be removed by reaction in the column, and the limited amount which could be removed overhead as the C.sub.4 -ethanol azeotrope, this scheme is likely to produce an excess of ethanol which must be removed with the bottom product ETBE, and would therefore be an undesirable impurity in the product.
The present invention provides a process and apparatus for the selective etherification of isobutene and overcomes the ethanol deficiency in the distillation reactor bed while providing additional temperature control in the straight pass reactor. A particular advantage of the present invention is that it allows the use of the 95% ethanol as a feed.